Notify waiting channels on completed transaction, not just the watermark

[misalcedo]

Description

This PR extends #1454 to notify waiting transactions not just when the watermark increases, but also when the transaction is completed.

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.

[Copilot]

Pull Request Overview

This PR ensures that waiting transactions are notified not only when the watermark increases but also when a transaction is completed. Key changes include:

• Extending the notification logic in coordinator.go to trigger on transaction completion.
• Updating the test suite in coordinator_test.go to simulate concurrent transactions using errgroup and randomize the number of inserts per transaction.
• Configuring additional connection limits in the test setup to support increased concurrency.

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

File	Description
go/logic/coordinator_test.go	Updated test logic for concurrent transaction simulation and setup.
go/logic/coordinator.go	Modified notification logic to include transaction completion conditions.